A piston for use in an internal combustion engine typically includes an insert about its circumferential extent. Grooves are formed in an outer radial face of the insert and are adapted to receive piston rings. The insert is generally formed from a ferrous alloy having a greater hardness and resistance to wear than the material of the piston body and piston head. However, the use of a ferrous alloy insert in a piston of a dissimilar metal such as aluminum results in unequal thermal expansion between the insert and the piston. As a result, a gap may be formed between the insert and the piston head that acts as a thermal barrier, preventing the transfer of heat from the insert during piston operation. Further, such a gap may result in undesirable localized stresses being applied by the piston on a corresponding cylinder wall, reducing engine life. Complete failure may occur if the insert separates from the piston.
One alternative to a ferrous metal insert is an insert formed of an alloy having increased hardness and wear resistance with a thermal expansion similar to that of the piston head and piston body. However, such alloys must be customized for a particular application, and are both difficult and expensive to develop. Further, the use of such an alloy does not eliminate a problem known as microwelding, wherein material from a piston ring and the insert are exchanged, bonding the ring to the insert. Such unwanted bonding may result in piston failure. Nor do such alloys typically provide any type of dry lubrication between a piston ring and an insert.
Another alternative to a ferrous metal insert involves the use of methods wherein material is applied in a customized fashion to a non-cast piston body and head and then machined to form an insert. The customized application of material to a non-cast piston is expensive, and subject to unreliability.